<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
            margin: 0;
            padding: 0;
        }

        html{
            font-size: 26.67vw;
        }
        div{
            width: 3.75rem;
            height: 100px;
            background: yellow;
        }

        /*
        100vw === 375px
        1px = 100/375=0.2667vw
        */
    </style>
</head>
<body>
    <div></div>
</body>
</html>